1. Technical Field
The invention is generally related to the field of wastewater treatment. More specifically, the invention relates to novel apparatus and methods of using same which address problems of organic pollutants, nitrogen, phosphorous, and/or coliform bacteria and other microorganism removal from residential, municipal, and/or industrial wastewater.
2. Background Art
A conventional method for reducing organic material, nitrogen and phosphorous in industrial wastewater is presented in FIG. 1. Prior art embodiment 40 comprises a source of screened, raw wastewater 42, a mixed only anoxic stage 44, an aeration stage 46, a clarifier 48, a filter stage 50, and a chlorination stage 52, the process producing a treated effluent at 54. A waste activated sludge holding tank 56 is also represented. Also illustrated in embodiment 40 are conduits 70, 72, and 74. Conduit 70 routes waste activated sludge from clarifier 48 to waste activated sludge holding tank 56, while conduit 74 returns waste activated sludge from clarifier 48 back to the mixed only anoxic stage 44. Finally, conduit 72 routs mixed liquor suspended solids return from an aeration stage 46 back to a mixed only anoxic stage 44. This process and apparatus does not utilize a high rate clarifier or a membrane stage.
U.S. Pat. Nos. 6,406,629 and 6,485,645 describe a wastewater treatment process for biologically removing phosphates incorporating a membrane filter. The process includes three zones, an anaerobic zone, an anoxic zone and an aerobic zone containing an anaerobic, anoxic and aerobic mixed liquor suspended solids (MLSS). Water to be treated flows first into the anaerobic zone. Anaerobic mixed liquor flows to the anoxic zone. Anoxic mixed liquor flows both back to the anaerobic zone and to the aerobic zone. The aerobic MLSS flows to the anoxic zone and also contacts the feed side of a membrane filter. The membrane filter treats the aerobic MLSS to produce a treated effluent lean in phosphorous, nitrogen, COD, suspended solids and organisms at a permeate side of the membrane filter and a liquid rich in rejected solids and organisms. Some or all of the material rejected by the membrane filter is removed from the process either directly or by returning the material rejected by the membrane filter to the anoxic or aerobic zones and wasting aerobic sludge. In a first optional side stream process, phosphorous is precipitated from a liquid lean in solids extracted from the anaerobic mixed liquor. In a second optional side stream process, anaerobic mixed liquor is treated to form insoluble phosphates that are removed in a hydrocyclone. While the patents do describe use of a membrane, there is no provision made for reducing the load of solids to the membrane, and no description of a high rate clarifier. In fact, conventional membrane bioreactors often must recirculate large flows from the membrane zone to the basins holding MLSS to help prevent excessive build-up of the retentate solids. The recirculation flow can be up to four (4) times the plant influent flow. Moreover, the patent does not describe a packaged system including a high rate clarifier.
U.S. Pat. Nos. 6,068,134; 6,274,044; and 6,454,104 disclose clarifiers useful in wastewater treatment. For example, the 104 patent discloses a clarifier in a sewage treatment process that includes efficient scum removal within the influent well, with discharge of the scum and other floatables as a dedicated waste stream separate from return activated sludge and biological scum collected in the clarifier. U.S. Pat. No. 5,972,220 discloses recycling of slurries in a wastewater treatment process. None of these patents discloses the use of a membrane or high rate clarifier.
U.S. Pat. No. 6,524,481 discloses a method and apparatus for cleaning a membrane module, where the module is employed in submerged fashion in a wastewater treatment facility. The membrane module typically has a plurality of porous membranes. The membranes are arranged in close proximity to one another and mounted to prevent excessive movement there between. Means are provided for entraining gas bubbles in a liquid flow such that, in use, the liquid and bubbles entrained therein move past the surfaces of the membranes to dislodge fouling materials there from. The gas bubbles are preferably entrained into the liquid using a venturi type device.
U.S. Pat. No. 6,165,359 discloses use of a second clarifier in a wastewater treatment facility. A high strength wastewater treatment system having a first tank with an inlet and an outlet, an aerator positioned within the first tank for passing oxygen into wastewater within the first tank, a second tank having a clarifier compartment positioned therein, an aeration device positioned in the second tank for passing oxygen into a liquid within the second tank, and a pipe connected to the first tank and the second tank for passing liquid from the second tank to the first tank. The second is interconnected to the outlet of the first tank. The second tank has an outlet extending from the clarifier compartment. In particular, the pipe has an end opening within the second tank and a diffuser connected to the pipe within the first tank. The diffuser is a venturi diffuser having a narrow section and a wide section. An air pump is connected to the venturi diffuser for injecting air into the narrow section. This delivery of air serves to draw liquid from the second tank through the pipe and into the first tank. The end of the pipe opens below the clarifier compartment in the second tank.
As discussed in U.S. Pat. No. 6,221,247, microfiltration and ultrafiltration are two recognized types of membrane separation processes. See Membranes: Learning a Lesson from Nature, Koros, W. J., Chemical Engineering Progress, Octuber 1995, pp. 68-80, the disclosure of which is incorporated herein by reference. These processes are known for such representative utilities as processing corn-stillage streams, concentrating emulsions and cell suspensions, reducing bacteria and particulate turbidity, recovering paint, removing oil microemulsion and separating biomolecules and virus from aqueous streams.
In microfiltration and ultrafiltration the mechanism for separation involves sieving of primarily liquid feed streams containing suspended species through a microporous membrane. The driving force for separation is a transmembrane pressure differential, i.e., the feed stream side is placed at a higher pressure than the filtrate stream side to force the liquid through the membrane pores. The transmembrane pressure gradient can be created by applying a pressure to the feed and/or by drawing a vacuum on the filtrate. Of course, suspended species of size larger than the membrane pores are rejected which yields a filtrate free of large species and a retentate stream concentrated in the rejected species.
Microfiltration and ultrafiltration suffer from the serious drawback that the membrane tends to foul over time in service. That is, as filtration continues the pores become blocked which reduces and ultimately stops the separation process until the foulant is cleaned if possible, or the fouled membrane is replaced with virgin membrane. Fouling of microporous membranes in microfiltration and ultrafiltration has been studied extensively. While the mechanisms and theories concerning fouling are very complex, two general categories have been identified, namely deposition and adsorption fouling phenomena. Deposition fouling occurs as a result of hydrodynamic forces. The pressure gradient across the membrane actively pushes the foulant species into the pores. Adsorption fouling relates to the adhesiveness between the foulant and the membrane. Generally, suspended species to be separated from the feed liquid that have great affinity for the membrane material tend to adhere to the membrane at the surface and in the pores. The bulk of foulant species settling on and in the membrane prevents further transmembrane flow of liquid.
There is a need in the wastewater treatment art for improved methods of employing membranes in wastewater treatment facilities, which feature reduced load on the membrane while maintaining or increasing the reliability and consistency of the membrane to function as an activated sludge clarifier, a tertiary filter, and to provide acceptable reductions in bacterial counts.